1. Field of the Invention
This invention relates to a photometric method and apparatus for analyzing a gas for its nitric oxide (NO) and nitrogen dioxide (NO.sub.2) content. More particularly, it relates to such a method and apparatus utilizing the conversion of NO to NO.sub.2 by oxygen.
2. Discussion of the Prior Art
The NO.sub.2 content of a gas has previously been photometrically determined using radiation of a wavelength falling within the broad NO.sub.2 absorption band. The nitrogen oxide (NO.sub.x) content of such gases, however, includes not only NO.sub.2 but also NO which does not absorb strongly in this wavelength band. A technique for analyzing a sample gas for its NO.sub.x content is described in U.S. Pat. No. 3,718,429, which issued on Feb. 27, 1973, to James A. Williamson, Jr. The method described in this patent proceeds by introducing the sample gas into a sample cell, introducing an oxygen-containing gas under pressure into the sample cell to bring about conversion of the NO to NO.sub.2, and then detecting the NO.sub.x content by observing the increase in absorption brought about by complete conversion of NO to NO.sub.2.
With normal NO.sub.x concentrations and oxygen pressures, the measurement of the NO.sub.x content of a gas by this procedure takes a relatively long period of time (on the order of 5 to 10 minutes), because the method requires complete conversion of NO to NO.sub.2. Making the NO.sub.x determination over such a time period normally does not create a problem. However, if a single photometric instrument is to be used to measure the NO.sub.x content of a number of gas sources, then, the cycle time for the measurement of each gas source may reach an unacceptable level.
It has now been found that it is possible to measure the total NO.sub.x content of the gas without waiting for omplete conversion of NO to NO.sub.2.